Domain structures and roles in bacteriophage HK97 capsid assembly and maturation

Head assembly in the double-stranded DNA coliphage HK97 involves initially the formation of the precursor shell Prohead I from similar to420 copies of a 384-residue subunit. This is followed by proteolytic removal of residues 2-103 to create Prohead 11, and then reorganization and expansion of the shell lattice and covalent cross-linking of subunits make Head II. Here, we report and structurally interpret solution Raman spectra of Prohead 1, Prohead 11, and Head 11 particles. The Raman signatures of Prohead I and Prohead 11 indicate a common alpha/beta fold for residues 104-385, and a strongly conserved tertiary structure. The Raman difference spectrum between Prohead I and Prohead 11 demonstrates that the N-terminal residues 2-103 (Delta-domain) form a predominantly alpha-helical fold devoid of beta-strand. The conformation of the Delta-domain in Prohead I thus resembles that of the previously characterized scaffolding proteins of Salmonella phage P22 and Bacillus phage phi29 and suggests an analogous architectural role in mediating the assembly of a properly dimensioned precursor shell. The Prohead 11 --> Head 11 transition is accompanied by significant reordering of both the secondary and tertiary structures of 104-385, wherein a large increase occurs in the percentage of beta-strand (from 38 to 45%), and a marginal increase is observed in the percentage of a-helix (from 27 to 31%). Both are at the expense of unordered chain segments. Residue environments affected by HK97 shell maturation include the unique cysteine (Cys 362) and numerous tyrosines and tryptophans. The tertiary structural reorganization is reminiscent of that observed for the procapsid --> capsid transformation of P22. The Raman signatures of aqueous and crystalline Head 11 reveal no significant differences between the crystal and solution structures.